Hypothetically, if there were some great deity, the one that rules over all, I would have this interchange with said deity:
Me: So you want me to kill all the children on that bus, correct?
Deity: Yes, because I demand it.
Me: So you mean to tell me that because you demand it, I should do it?
Deity: Yes! Go forth and spill the blood of those unbaptized children!
Me: May I ask you a question?
Deity: *clearly exasperated* Yes...what is it?
Me: Prove to me that you aren't just some figment of my imagination, or some alien screwing with me.
Deity: I prove myself to no one. I am the almighty creator, the architect of this grand existence!
Me: *yawns* But by showing yourself to me, does that not prove that you are at least in existence, and by your own claims, proof is not conducive to
Me: Well, are you going to answer?
Deity: *hologram disengages revealing a gray alien* You win.
I then proceed to pull out a baseball bat and bash the little bugger's skull in. Case closed.
Even if it were THE "God" I'd tell him to get bent and then use circular logic for the next hour to make him/her/it really ticked off.
Thanks to Douglas Adams for giving me an arguement in case I ever run into a deity.